ef code first - Entity Framework: I set the foreign key, SaveChanges then access the navigation property, but it doesn't load the related entity. Why not?

Question

I am using this Entity class with Entity Framework 5 Code First:

public class Survey
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    public string SurveyName { get; set; }

    [Required]
    public int ClientID { get; set; }

    [ForeignKey("ClientID")]
    public virtual Client Client { get; set; }
}

And in my Controller's Create method I do this:

    Survey entity = new Survey()
    {
        SurveyName = "Test Name",
        ClientID = 4
    };
    db.Surveys.Add(entity);
    db.SaveChanges();
    Client c1 = entity.Client;                    //Why is this null?
    Client c2 = db.Clients.Find(entity.ClientID); //But this isn't?

    string s2 = c2.ClientName;
    string s1 = c1.ClientName;   //null reference thrown here

The Client navigation property remains null after SaveChanges. I expected the call to load the Client from the database because the foreign key exists. Why didn't it do that?

EDIT The code here comes from when my controllers were dependent on DbContext. Not long after I got this working I re-factored the code to use repositories and a unit of work. Part of that move was driven by the fact that it just felt wrong to use Create when I wanted to use new. What happened then was that I hit a problem with how to ensure proxies are created when using the repository pattern.

Answer 1

To ensure that lazy loading of a navigation property will work after you've created the parent you must not create the Survey with the new operator but create it by means of the context instance because it will instantiate a dynamic proxy that is capable to lazily load the related Client. That's what the DbSet<T>.Create() method is for:

Survey entity = db.Surveys.Create();

entity.SurveyName = "Test Name";
entity.ClientID = 4;

db.Surveys.Add(entity);
db.SaveChanges();

Client c1 = entity.Client;
string s1 = c1.ClientName;
// will work now if a Client with ID 4 exists in the DB

Just to emphasize: It's not the line entity.ClientID = 4; or db.Surveys.Add(entity); or db.SaveChanges that loads the client from the DB, but the line Client c1 = entity.Client; (lazy loading).